Light propagating within an optical fiber may undergo chromatic dispersion, i.e. different wavelengths of the light may travel at different group velocities leading to varying wavelength-dependent delays in transmission. The chromatic dispersion imparted by an optical fiber may be characterized by: (1) the dispersion at one specific wavelength, and (2) the dispersion slope. The dispersion slope indicates the extent to which dispersion varies as a function of wavelength.
The dispersion slope of an optical fiber can significantly limit the usable bandwidth for a wavelength-division multiplex (WDM) system, which uses multiple information channels each having their own wavelength of light. Each information channel can accumulate its own amount of dispersion on the transmission length. For example, in a WDM system having 10 Gb/s data-rate information channels, the information channels can accumulate a large amount of dispersion over long transmission distances, such as transoceanic transmission distances (e.g., 7000-10,000 km). When the accumulated dispersion is too large, the system performance utilizing On-Off-Keying (OOK) modulation format is degraded due to intersymbol interference which in turn limits the system bandwidth.
Various dispersion management techniques have been used to manage dispersion. One dispersion management technique involves dispersion mapping where optical fiber types are selected and arranged to manage the dispersion in the transmission segments of an optical communication system. One example of a transmission segment design mixes spans of non-zero dispersion-shifted fiber (NZDSF) or spans of dispersion flattened fiber (DFF) having a non-zero dispersion with spans of dispersion compensation fiber (DCF) to realize periodic dispersion compensation over the length of the optical transmission segment. The length of each period in such periodic dispersion maps may be in the range of about 500 km per period.
For conventional systems utilizing OOK modulation formats, such dispersion mapping techniques have been useful in maintaining a low end-to-end path average dispersion and suppressing fiber nonlinearities. It has been recognized, however that that differential phase-shift-keying (DPSK) modulation formats can provide advantages over OOK. In DPSK modulation formats ones and zeros are indicated by differential phase transitions. DPSK formats include Return-to-Zero DPSK (RZ-DPSK), wherein a return-to-zero amplitude modulation is imparted to a DPSK signal, and Chirped-Return-to-Zero DPSK (CRZ-DPSK). Compared to OOK, RZ-DPSK modulation may, for example, provide a potential 3 dB reduction in the required optical signal-to-noise (OSNR) for a particular bit error rate (BER) when using a balanced receiver and reducing cross-phase modulation (XPM) penalties.
Accordingly, there is a need for dispersion management in optical networks using DPSK modulation formats.